1. Field of the Invention
This invention relates to methods and systems for sensing rotor position. More specifically, the invention is a position sensing system and method for sensing the position of a rotor in hybrid stepper motor.
2. Description of the Related Art
Stepper motors are used in a wide variety of machines to provide digital control of moving parts. However, typical stepper motors exhibit several undesirable characteristics. Principal among these characteristics are: insufficient resolution due to a relatively low number of steps per revolution (e.g., 200 steps per motor revolution); a high ripple torque; and a high constant internal power dissipation regardless of motor load. While the resolution and ripple torque problems can be addressed by providing high resolution motor command signals, the problem of constant internal power dissipation has been a continuing source of power loss.
The internal power dissipation of stepper motor is a function of motor current I and the resistance R of a motor winding. Accordingly, if the stepper motor is driven by a constant current source, power dissipation (or I2R) will remain constant regardless of the load on the stepper motor.
As a result, the stepper motor generates heat associated with the fully-loaded condition even when the motor is only lightly-loaded. The unnecessary production and build-up of heat can be detrimental to the motor itself as well as surrounding components.
U.S. Pat. No. 6,013,999 discloses a stepper motor control system that reduces internal power dissipation losses. The system includes Hall position sensors that produce position feedback signals which are sine and cosine waveforms of the form A sin (X+θ) and A cos(X+θ), respectively, that are in phase with the back “electromagnetic force” (emf) of the stepper motor. The system is designed to work with brushless servo stepper motors that use a rotor having alternating north and south poles. The position sensors are placed at positions relative to the motor's stator. More specifically, the sin(X+θ) signal is measured at a position that is located 90 electrical degrees away from one stator winding while the cos(X+θ) signal is measured at a position located 90 electrical degrees away from another stator winding.
The above-described type of position sensing only works well for a rotor having alternating north/south pole pairs. That is, it does not work for another prevalent type of stepper motor known as a hybrid stepper motor. The rotor of a conventional hybrid stepper motor is illustrated in FIG. 1 and is referenced generally by numeral 10. Briefly, rotor 10 has armatures 12 and 14 mounted on a shaft 16 in a spaced-apart fashion. Each of armatures 12 and 14 is typically made from a magnetic or magnetizable material (e.g., soft iron). One armature (e.g., armature 12) defines north pole “steps” at its radial perimeter while the other armature (e.g., armatures 14) defines south pole steps at its radial perimeter. Since the rotor does not have alternating north and south poles, the positioning sensing approach disclosed in U.S. Pat. No. 6,013,999 cannot produce the required A sin(X+θ) and A cos(X+θ) position signals.